1. Field of the Invention
The invention concerns a digital device for controlling the amplitude and the phase of a radio frequency signal, a pre-distortion linearizer including at least one such device and a digital method of controlling the amplitude and the phase of a radio frequency signal.
The invention applies in particular, although this is not limiting on the invention, to high-frequency pre-distortion linearizers used on the input side of travelling wave tubes (TWT) or solid state power amplifiers (SSPA) to linearize them.
2. Description of the Prior Art
The skilled person is well aware that it is necessary to use a linearizer to obtain good electrical efficiency of an amplifier, in particular a power amplifier, by having it operate near saturation. Near this operating point the linearity of the amplifier is strongly degraded relative to the linearity of the same amplifier used with signals of lower amplitude than those required to saturate the amplifier.
A non-linearity corrector device can be used to increase the dynamic range of an amplifier in linear operation without sacrificing the electrical efficiency obtained near saturation.
At microwave frequencies it is preferable to use a predistortion linearizer. This technique employs a coupler to extract a portion of the signal before it is amplified and which is then processed by various active and passive electronic components to obtain a non-linear correction signal having the same non-linearity characteristics as the wanted signal to be corrected but with the opposite phase. The predistortion linearizer supplies a correction signal with the signal to be amplified at the input of the microwave power amplifier whose non-linearities are to be corrected.
Predistorion linearizers of the above kind conventionally comprise a so-called linear channel including an amplifier operating with low level signals and a so-called non-linear channel including an amplifier generating non-linear distortion of the same amplitude as that of the power amplifier to be linearized.
For such linearizers it is necessary to adjust the phase difference and the amplitude difference of the signals propagated in the two channels.
Correct operation of the linearizer, i.e. obtaining correct pre-distortion, is conditioned by very fine control of the amplitude and phase differences.
The amplitude and the phase are controlled by analog or digital control circuits.
Analog control circuits achieve a high accuracy, in the order of one degree of phase and in the order of 0.1 dB of amplitude. However, they have the usual drawbacks of analog technology, in particular they are particularly sensitive to temperature variations, to input power level variations, and to control voltage variations and do not allow automation of setting up and measurement operations.
Furthermore, in most applications the circuits on the output or input side are digital circuits so that an analog/digital converter is required to connect them.
In the case of digital control circuits the use of a variable digital phase-shifter to control the phase connected in series with a variable digital attenuator for controlling the amplitude is known in itself. Accordingly, in the case of predistortion linearizers the association of an M-bit phase-shifter providing complete coverage of the phase plane (360.degree.) and an N-bit attenuator having an amplitude control dynamic range of A dB achieves total coverage of 360 degrees of phase and A dB of amplitude with a resolution of 360.degree./2.sup.M in phase and A/2.sup.N dB in amplitude. The number of control states obtained on an amplitude-phase plane is 2.sup.M+N and they are uniformly distributed. FIG. 1 shows a distribution of the above kind in the case where M=N=5.
However, control accuracy is limited because it is directly related to the achievable value of the least significant bits of the attenuator and the phase-shifter.
For example, in the case of standard 6-bit digital phase-shifters providing complete coverage of the phase plane (360.degree.) the greatest accuracy that can be achieved is 5.625.degree.. Similarly, in the case of standard 6-bit digital attenuators having an amplitude control dynamic range of 32 dB the greatest accuracy that can be obtained is 0.5 dB.
To improve control accuracy, consideration might be given to increasing the number of bits available by dividing the value of the least significant bit. For example, an 8-bit phase-shifter would be four times more accurate than a 6-bit phase-shifter. However, currently available technology, especially in the microwave domain, cannot reproducibly provide digital circuits having a sufficient number of bits to obtain such accuracy.
Furthermore, the cost of any specific implementation of such a circuit is prohibitive.
The aim of the invention is therefore to alleviate the drawbacks previously mentioned.
It concerns a device and a method for controlling a radio frequency signal achieving an accuracy at least equivalent to that achieved by analog circuits combined with the advantages of digital technology, including:
temperature stability; PA1 operation over a wide frequency range; PA1 very low sensitivity to unwanted variations in control voltages due, for example, to aging or to electromagnetic interference; and PA1 simple automation of setting up and measurement operations. PA1 a) dividing the radio frequency signal into a first signal part and a second signal part; PA1 b) digitally controlling the amplitude of the first or second signal part; PA1 c) digitally controlling the phase of the second or first signal part; and PA1 d) recombining the controlled signal part to form a radio frequency signal with controlled phase and amplitude.